[lld] Set up .arcconfig to point to new Diffusion LLD repository

Summary:
We want to automatically copy the appropriate mailing list
for review requests to the LLD repository.

For context, see the proposal and discussion here:

http://lists.llvm.org/pipermail/cfe-dev/2017-November/056032.html

Similar to D40179, I set up a new Diffusion repository with callsign
"LLD" for lld:

https://reviews.llvm.org/source/lld/

This explicitly updates lld's .arcconfig to point to the new C
repository in Diffusion, which will let us use Herald rule H270.

Reviewers: peter.smith, ruiu, rafael, sammccall

Reviewed By: sammccall

Subscribers: dlj, bkramer, sammccall, klimek

Differential Revision: https://reviews.llvm.org/D40495

llvm-svn: 319955